1. Field of the Invention
The invention relates to a finishing device to lay and compact asphalt layers and a method for operating the device.
2. The Prior Art
Asphalt finishers are known that consist of a receiving bucket, for the temporary storage of the hot asphaltic mixture, and conveyor belts, for the longitudinal transport of the asphalt before the laying beam. Spreader screws are provided for demixing-free transverse distribution of the mixture across the laying width and a laying beam for pre-compaction and striking off the asphalt. The laying beam is suspended on a traction vehicle in an articulated manner and floats upon the mixture to be laid.
Conventional laying beams consist of a tamping beam (tamper) and vibrating beam (screed plate).
The newer high compaction beams contain additional compacting elements in order to increase the level of pre-compaction. Depending on the effectiveness of the laying beams, rollers may be used for recompacting.
Thus, a layer is laid of the mixture in the receiving bucket in the specified thickness. The amount of compaction is a deciding factor in the mechanical properties and the durability of the asphalt. Higher compaction means a significant improvement in quality with the correct mixture conception and a suitable course structure. The continuous growth in traffic and the increase in axle loads requires a high degree of compaction.
According to German Patent No. 90 13 760.4 U1, a road building machine for renewing road surfacing is disclosed, which consists of heating aggregates of a milling unit provided with a drive unit, a mixing unit and a conveyance device, and drawing off new material from a material trough, as well as a laying unit. The conveyance device consists of two belt-conveyors arranged in tandem in the lengthways direction of the road building machine. The first conveyor belt extends from the material trough to a transfer unit disposed between the tractive machine and the trailer and the second conveyor belt extends from the transfer unit into the region of the laying units.
In the case of these finishers, the existing asphalt is heated by heating aggregates which are then reamed, distributed or fed into a mixing region and then distributed. Because the bitumen is a relatively poor heat conductor, temperatures of 300xc2x0 to 600xc2x0 C. or more are required to heat the approximately 4 cm thick top region. The use of these finishers working in combination with heating aggregates thus leads to significant environmentally degrading emissions, owing to large temperature differences within the asphalt. Thereby the binder is modified and the job site mixture is subject to a series of factors, which lead to considerable fluctuations in quality within a section under construction compared with production at an asphalt mix plant. In the case of these finishers, it is also disadvantageous that a certain dwell time is required to heat the lower courses, which is also dependent on the weather to a great extent. Consequently, only a low working speed is possible and the course thickness of the asphalt to be distributed or changed is limited.
The disadvantage of the known finishers is that they only enable the laying of a delivered type of asphalt mixture. Surface and binder courses are laid in relatively thin coats. Asphalt compacting depends largely upon the thermal capacity of the asphalt layer. This is closely related to the layer thickness, the weather conditions, and the temperature of the mixture as delivered on the job site.
Rapid asphalt temperature losses lead to difficulties during compacting, to insufficient bonding between layers and increased voids content diminish the quality. In numerous studies, it has been proven that there are manifold deficiencies in the compacting of relatively thin rolled asphalt surface layers. Necessitated by the sequence of construction operations, allocation of funds, and unpredictable weather influences, asphalt surfaces are often laid during unfavorable weather conditions. Raising the mixture temperature in the delivery state is subject to limits as it causes increasing oxidation of the binder, which in turn worsens the compaction ability and is generally disadvantageous. Generally, one thus strives to lower mixture temperatures rather than increasing them.
It is the object of the present invention to provide a finisher which enables high compaction of the asphalt without increasing the processing temperature and without increasing compaction expenditures.
This object is accomplished by providing a method in which the material is transported simultaneously from two separate receiving buckets attached to the machine, via two independent conveyance systems, to respective distribution devices. The distribution devices are arranged in tandem, staggered in the direction of operation, the devices deposit the material in superimposed layers and lay it.
The finisher according to the invention is provided with a receiving device for the temporary storage of the hot mixture. The receiving device consists of two separate receiving buckets, from each of which a mixture transport system with at least one conveyance device leads to the distribution devices, and the distribution devices are constructed as spreader screws.
The finisher according to the invention is advantageous over the prior art in the following ways. In practice, it has been demonstrated that when laying thicker courses, better degrees of compactness are achieved with the same technology. The finisher according to the present invention allows the simultaneous laying of two different hot mixtures directly upon each other. With the increase in layer thickness, the thermal capacity of the laid asphalt is considerably increased so that laying may even be carried out during unfavorable weather conditions.
Moreover, the finisher according to the invention enables the simultaneous laying of two supplied asphalts, different in composition, in hot work method, which are produced in an asphalt mixing plant under strictly controlled qualitative and environmentally relevant conditions.
Furthermore, it is advantageous that, through direct hot-on-hot laying, an optimum bonding between the two layers is achieved. The otherwise conventional use of asphaltic emulsions to bind the asphalts is not required. The considerably greater thermal potential also effects a better bonding with the already laid asphalt courses.
In contrast to the recycling finishers, the finisher according to the present invention enables a high working speed. The asphalt delivered from the mixing plant has fewer fluctuations and a defined temperature. Furthermore, the aforementioned hazardous emissions are avoided, as production is effected at an even and low temperature.